The detection of analytes including proteins, DNA/RNA and metabolites from body fluids and other samples of biological origin is essential for a variety of applications including medical testing, toxin detection and forensic analysis. Improved, point-of-care testing of such analytes is an urgent worldwide requirement (Yager, P.; Domingo, G. J.; Gerdes, J., Point-of-care diagnostics for global health. Annu Rev Biomed Eng 2008, 10, 107-44). The current systems designed for such applications suffer from several drawbacks such as high costs, bulkiness and delayed results. There is therefore a large unmet need for the development of systems that are low-cost, portable, convenient to handle and show high efficiency towards detection. These systems should also be capable of rapidly identifying a broad range of analytes from samples of biological origin.
Microfluidic, lab-on-a-chip methods have gained prominence over the past decade as solutions to some of these problems. However, existing technologies for the manufacture of microfluidic lab-on-a-chip devices are handicapped by the absence of mature manufacturing processes that can enable the transition of ideas from academic labs to industry. Adaptation of traditional methods used for microelectronic fabrication for this purpose meant that early microfluidic devices were synthesized in glass or silicon. However, these are materials that require expensive processing conditions and high capital investment.
To address this problem, a number of different materials and processing methods have been explored for the fabrication of microfluidic devices (Becker, H.; Locascio, L. E., Polymer microfluidic devices. Talanta 2002, 56 (2), 267-287). These materials include plastics such as PDMS (polydimethylsiloxane) (McDonald, J. C.; Whitesides, G. M., Poly(dimethylsiloxane) as a material for fabricating microfluidic devices. Acc Chem Res 2002, 35 (7), 491-9), PMMA (polymethylmethacrylate) (Klank, H.; Kutter, J. P.; Geschke, O., CO(2)-laser micromachining and back-end processing for rapid production of PMMA-based microfluidic systems. Lab Chip 2002, 2 (4), 242-6) and COC (cyclicolefin copolymer) (Pu, Q.; Oyesanya, O.; Thompson, B.; Liu, S.; Alvarez, J. C., On-chip micropatterning of plastic (cylic olefin copolymer, COC) microfluidic channels for the fabrication of biomolecule microarrays using photografting methods. Langmuir 2007, 23 (3), 1577-83). Plastics are relatively cheap and they have advantages such as their processability, transparency and the ability to form intricate patterns down to the micron scale. However, they also suffer from some disadvantages such as their natural hydrophobic nature which precludes simple capillary flow, their carbon footprint and the lack of mature manufacturing methods that are easily adaptable for large scale microfluidic plastic chip fabrication. Further, for the plastic-based microfluidic devices, sophisticated and expensive readers that can direct fluid flow and can provide a read-out from the plastic chip are still required, which renders the entire device and operation unsuitable for very low-cost and robust point-of-care diagnostics.
On the other hand, paper-based lateral flow immunoassays (LFIs) have been hugely successful in the market place with a variety of rapid tests such as home pregnancy tests being widely available. Visual readouts in the form of a color change are used for detection while sample flow occurs automatically through capillary action. Further, mature manufacturing processes are already available for such devices. However, LFIs come with a set of disadvantages too. They are not very reliable and do not provide for the ability to perform multiplex tests. One of the reasons for this is the lack of an ability to define a ‘flow-path’ in a paper based device (Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M., Patterned paper as a platform for inexpensive, low-volume, portable bioassays. Angew Chem Int Ed Engl 2007, 46 (8), 1318-20).
Recently, the Whitesides group advanced such technology by patterning paper into selectively hydrophilic and hydrophobic portions. A patterned flow field can therefore be defined. However, paper-based devices still have some problems like the lack of mechanical stability and the absence of low-cost manufacturing methods that can deposit multiple reagents without heat treatment or exposure to high stress. Very recently, cotton thread has also been explored as a medium for microfluidic chip fabrication (Li, X.; Tian, J.; Shen, W., Thread as a Versatile Material for Low-Cost Microfluidic Diagnostics. ACS Applied Materials & Interfaces 2009, 2 (1), 1-6). Experiments were performed on single cotton threads or cotton threads that have been sewed onto a plastic substrate and color change based readouts were used to detect the presence of a reagent. These experiments are not necessarily conducive towards development of high-throughput and reproducible methods for manufacture of point-of-care diagnostic devices using either the cotton fibers or other suitable materials. Hence, there remains a dire need in the art that addresses all the problems associated with diagnostic devices, their manufacture, cost and reliability.